Not all varieties of Rhus will reliably produce red leaves in autumn - some only do yellow and orange, some yellow and orange with occasional red. If you want fiery red leaves in autumn, the variety to choose is 'Red Autumn Lace'. Also, autumn leaf colour is,to some extent, dependent on weather conditions - the best autumn colours usually occur when we have a late Indian type summer, and then it gets sharply cold, when all the leaves colour rapidly and then fall quite quickly. You haven't any stag horns because it's not flowered, basically. It may do so later, as it matures.

Heat and drought in the summer cripples fall color, too. I understand much of the UK had such weather for a signifigant part of the growing season this year.